The invention provides a connecting structure of a circuit board, a connecting part of the circuit board, and an electronic device capable of alleviating a temperature difference between the connecting parts under hot pressure welding. A connecting structure 10 of a circuit board comprises a first circuit board 11 and a second circuit board 12, with a first connecting part 15 and a second connecting part 16 opposedly disposed via an adhesive 13. The first connecting part 15 and the second connecting part 16 are pinched by a pair of pressurizing jigs 20 and subjected to hot pressure welding so that first circuit patterns 17 and second circuit patterns 18 are in contact with each other. In the connecting structure 10 of the circuit board, the first circuit board 11 is a soft base material 21, and a heat-insulating layer 28, having lower thermal conductivity than the soft base material 21, is provided on only a part 27 of the region corresponding to the first connecting part 15 on the rear surface 21B of the soft base material 21.